1. Field of the Invention
The present invention relates to position detection systems, position detection methods therefor, position detection communication devices, and communication devices.
2. Description of the Related Art
In these years, among communication devices that communicate with each other to perform prescribed processing are, for example, vehicle-mounted and mobile devices for vehicles having a passive keyless entry system mounted therein.
Communication between the vehicle-mounted device and the mobile device for a vehicle with a passive keyless entry system installed is described in detail below. The vehicle-mounted device is installed, for example, in the door on the driver-seat side of the vehicle, and the mobile device is carried by the owner (hereinafter referred to as the wearer) of the vehicle concerned. After stopping the vehicle engine, the wearer opens the door and exits the vehicle.
The vehicle-mounted device transmits a signal (hereinafter referred to as signal A) in order to determine whether or not the mobile device is within a range (area) where communication between the vehicle-mounted device and the mobile device is feasible. When the wearer is within a communication-feasible range, the mobile device receives the signal A from the vehicle-mounted device, and transmits a signal B in response to this signal A. When the vehicle-mounted device receives the signal B from the mobile device, it determines that the mobile device is within the communication-feasible range. Transmission of the signal A from the vehicle-mounted device is repeated at prescribed intervals.
If the wearer goes outside the communication-feasible range, the mobile device is no longer able to receive the signal A from the vehicle-mounted device. As a result, the vehicle-mounted device can no longer receive the signal B from the mobile device in response to the signal A. When, for example, the vehicle-mounted device no longer receives the signal B from the mobile device within a predetermined time period, it transmits an instruction signal to a controller, separately installed inside the vehicle, to lock the doors of the vehicle. The controller locks the doors of the vehicle, according to the instruction signal from the vehicle-mounted device. Thus, when the wearer moves away from the vehicle and the mobile device is outside the communication-feasible range, the doors of the vehicle are locked.
Next, in cases where, having moved outside the communication-feasible range, the wearer returns to within the communication-feasible range, the mobile device again receives the signal A from the vehicle-mounted device. The mobile device transmits the signal B. When the vehicle-mounted device receives the signal B from the mobile device, in order to determine whether or not the mobile device relates to this vehicle, it transmits a read signal, for example, to read information from the mobile device. The mobile device transmits mobile device information, according to the read signal from the vehicle-mounted device. The vehicle-mounted device determines whether or not the mobile device relates to this vehicle, based on the mobile device information from the mobile device. When the vehicle-mounted device determines that the mobile device relates to this vehicle, it transmits an instruction signal to the above mentioned controller to unlock the doors of the vehicle. The controller unlocks the doors of the vehicle, according to the instruction signal from the vehicle-mounted device.
In this way, for a vehicle with a passive keyless entry system installed, it is possible to lock or unlock the doors of the vehicle without inserting a vehicle key into a keyhole, by performing communication between the vehicle-mounted device and the mobile device (refer to Japanese Laid-open Patent Publication 2000-198420).
However, with communication between the vehicle-mounted device and the mobile device for the above-mentioned vehicle with the passive keyless entry system installed, there has been a possibility that the vehicle may be stolen when a so-called relay attack is carried out. In this relay attack, in a state where in normal circumstances communication is not possible between the vehicle-mounted device and the mobile device due to the wearer moving outside the communication-feasible range, there is a theft ploy in which, by utilizing a relay unit, communication between the vehicle-mounted device and the mobile device is made possible, a door of the vehicle is unlocked, and the vehicle is stolen.
The relay attack is detailed below, using FIG. 9. FIG. 9 illustrates communication between the vehicle-mounted device 101 and the mobile device 102, utilizing relay units A and B. The vehicle-mounted device 101 is installed, for example, in the door on the driver-seat side and transmits the above described signal A. Within the communication range indicated by a dashed line C, the vehicle-mounted device 101 can transmit and receive signals. When the mobile device 102 receives the signal A from the vehicle-mounted device 101 as described above, it transmits the signal B in response to the signal A. Moreover, within the communication range indicated by a dashed line D, the mobile device 102 can transmit and receive signals.
When the wearer moves outside the communication-feasible range, the signal B from the mobile device 102 in response to the signal A can no longer be transmitted to the vehicle-mounted device 101 as described above, and the controller (not shown) locks the doors according to an instruction signal from the vehicle-mounted device 101.
At this time, it is supposed that intermediaries x and Y, who are attempting to steal the vehicle, are present. The intermediary X holds a relay unit A and enters the communication range C of the vehicle-mounted device 101. Furthermore, the intermediary Y, holding a relay unit B, moves close to the wearer and the relay unit B enters the communication range D of the mobile device 102. Since the relay unit A held by the intermediary X is within the communication range C of the vehicle-mounted device 101, it receives the signal A; the relay unit A detects the signal A, amplifies it and transmits it. Since the relay unit A amplifies the signal A, the signal A can be transmitted over a wider range than the communication range C of the vehicle-mounted device 101. When the signal A, amplified by the relay unit A, is received by the relay unit B, held by the intermediary Y, the relay unit B detects this amplified signal A, attenuates it to, for example, the level the signal A had before being amplified by the relay unit A, and transmits it. At this time, since the relay unit B is within the communication range D of the mobile device 102, the mobile device 102 receives the signal A, attenuated by the relay unit B. The mobile device 102 takes the signal A as though it was transmitted by the vehicle-mounted device 101, and transmits the signal B in response to this signal A. When the relay unit B receives the signal B, the relay unit B detects the signal B, and transmits it after amplification. Since the relay unit B amplifies the signal B, the signal B can be transmitted over a wider range than the communication range D of the mobile device 102. When the relay unit A receives the signal B, amplified by the relay unit B, the relay unit A detects this amplified signal B, and attenuates it to, for example, the level the signal B had before being amplified by the relay unit B, and transmits it. At this time, since the relay unit A is within the communication range C of the vehicle-mounted device 101 as described above, the vehicle-mounted device 101 receives the signal B that was attenuated by the relay unit A. As a result, since the vehicle-mounted device 101 receives the signal B in response to signal A, it determines that the mobile device 102 is within the communication-feasible range. The vehicle-mounted device 101 performs the above described processing to unlock the doors of the vehicle. The intermediary X, for example, gets into the unlocked vehicle, and the vehicle is stolen.
In this way, regarding communication between the vehicle-mounted device 101 and the mobile device 102 with the relay units A and B intervening, since the vehicle-mounted device 101 receives the signal B via the relay units A and B, there was a possibility that it would determine that the mobile device 102 is within the communication-feasible range. As a result, there was a possibility that the vehicle-mounted device 101 would transmit an instruction signal to the controller to unlock the doors of the vehicle and, regardless of the fact that the wearer is not within the communication-feasible range, the controller would unlock the doors.